The present invention relates to a position measuring apparatus which can be utilized in a working machine for working the surface of an object, a measuring machine for measuring the surface state of an object or the like for a purpose of controlling the position of the object or other purposes.
The present invention also relates to an electron beam lithography system to which the position measuring apparatus is applied.
In the semiconductor aligner of the prior art, a desired circuit pattern on a pre-formed original pattern (reticle or mask) is transferred onto a wafer after the circuit pattern has been aligned with an exposure area on the wafer. Such a transferring device is a high-precision reduction projection aligner and fixed to a high-precision X-Y stage on the side of wafer for exposing the entire wafer surface to be transferred. The transferring device is called "stepper" when it is used to perform the step/repeat action relative to the optical wafer system.
As the integration of LSI recently increases, the circuit line width required by the semiconductor is more and more reducing. The rate of reduction in the prior art stepper is mainly 1/5. It has been said that patterns equal to or less than one .mu.m could not be resolved due to the past limits of wavelength. However, patterns in order of sub .mu.m can be resolved by the use of an improved optical or illumination system, a phase shift mask for regulating the phase of the light on the reticle or the like. With improvement of the resolution, the focal depth of the reducing glass has been smaller. Further, the precision required by the transfer of the pre-formed original pattern onto the wafer has been more strict. It is thus required that the positions of a specimen relative to the alignment optical system of the stepper in the directions of specimen surface and focus are detected with higher precision.
The phase shift mask is adapted to reduce the phase of a light passing through the light transmission portion of the original pattern or the optical distance of the light transmission portion by etching the thickness of a substrate or otherwise to change the optical distance through a technique of adding a material of different refractive index or any other technique. Therefore, the phase shift mask is prepared by a method of selectively cutting the substrate at a particularly light transmission part in the direction of thickness to change the optical distance or a method of again applying a resist onto the substrate on which the pattern has already been drawn and adding a material for partially changing the phase of the light transmission part before any unnecessary part is etched out. In any event, the pattern drawing device must be used to expose the resist at a given location with high precision.
Prior to detection of an alignment mark on the mask, the position of an exposing beam is measured and calibrated by the use of a mark on the stage or the like. Since the position of the stage is precisely monitored by the laser interferometer, the position and displacement of the beam can be measured based on the coordinate system in the laser interferometer.
If the interior of the body tube in an electron beam aligner is contaminated, however, a drift may be created in the electron beam due to charge-up. This may vary measurements. Since the position of the mark on the mask is measured indirectly through the mark on the stage, it cannot be judged whether the variations in the measurements is caused by the beam drift or by the displacement of the mark itself.
In the electron beam lithography system and stepper, it may be considered that a measurement optical system using a laser beam is used as means for precisely measuring the position of a mark without affection of any drift. However, the internal space of the electron beam lithography system or stepper is substantially occupied by its inherent drawing optical system. It is often found that an internal space required to arrange the optical system for measuring the position of an object to be worked is limited.
Particularly in a system of such a type that a laser beam is irradiated onto a given diffraction pattern to form a diffracted light which is in turn used to obtain the information relating to the position of the object, the diffracted light required to measure the position of the object intends to be blocked by a member or members required to work the object in the drawing optical system, projection optical system or the like, since the diffracted light at the diffraction pattern is spread out.
There is a further need of providing an electron beam lithography system to which a small-size and high-precision position measuring apparatus is applied.
It is therefore an object of the present invention to provide a position measuring apparatus which can use a diffracted light obtained through less space to measure the position of an object.
Another object of the present invention is to provide an apparatus which can precisely measure the position of an object and particularly its position in X and Y directions.
A further object of the present invention is to provide a small-size and high-precision electron beam lithography system comprising a position measuring apparatus which can precisely measure the position of an object.